GB2325786A - Phase switch with slotline - Google Patents

Abstract

A phase switch comprises microstrips 66, 74, 86 arranged to cross slotline 72 formed between opposing portions 68, 70 of a ground plate, microstrips 74, 86 being connected at one end via diodes 76, 88 to respective ground plate portions and at the other end to a common outlet port 98. When diode 76 is conducting, (diode 88 non-conducting) a signal inputted to strip 66 is outputted via strip 74, the output phase being shifted 180‹ since strips 66 and 74 are earthed on opposite sides of the slot. Conversely, when diode 88 is conducting, (diode 76 non-conducting) a signal inputted to strip 66 is outputted via strip 86, the output phase being shifted 0‹ since strips 66 and 86 are earthed on opposite sides of the slot.

Description

PHASE SWITCH The present invention concerns a phase switch, particularly a broad band, low insertion loss phase switch with low phase dispersion over the operating band width. The invention has particular, but not exclusive, application to use in digital portable and cordless telephones.

In the field of portable telephones, particularly portable digital telephones, small size, long battery life and fiexibility are key requirements. There are three well-known portable telephone systems, the Group Special Mobile (GSM900 or GSM), the Personal Communications Network (DCS1800 or PCN) and the Digital European Cordless Telephone (DECT). The GSM system operates at around 900 megahertz (MHz) while the PCN system operates at around 1800 megahertz (MHz).

In order to provide an economical telephone handset, some form of switching is required between transmission and reception of signals for the following reason. When the handset is operating to transmit signals, a signal having a power as high as 2 Watts is transmitted from a transmitter circuit via an antenna. In receive mode, however, the handset must be capable of picking up a signal whose strength is as low as -106dBm. If the power levels produced by the transmitter circuit were to be applied or leaked to the receiver circuit there is a strong possibility that permanent damage to the receiver circuit would occur. Leaked signals can also affect the ability of the receiver to pick up low level signals, i.e. its sensitivity. In order to avoid this, telephone handsets are typically provided with a circuit known as a transmit/receive switch.

This circuit switches the antenna of the handset to either the transmitter circuitry or receiver circuitry as appropriate. In this way, the power of the transmitter circuit is never applied across the sensitive input to the receiver circuitry. Several transmit/receive switch arrangements are known, having varying degrees of complexity.

While known transmit/receive circuitry exists, the bandwidth over which such circuitry operates is limited. For example, the GSM frequency band starts at 880 MHz and goes up to 960 MHz. This represents a bandwidth of less than 10% (the range in frequency divided by the value of the lower frequency). Typical PCN systems operate from 1710 MHz to 1880 MHz. Again, this is a bandwidth of less than 10%. Known transmit/receive switches can give adequate performance over either, but not both, of these ranges. The problem thus arises that there is no cost effective way in which an antenna of a portable handset can be connected to either the transmitter or receiver of a portable telephone for operation both in the GSM waveband and the PCN waveband. The frequency bandwidth is over 100% (also said to be greater than one octave).

Many transmit/receive switch arrangements rely on the operation of a phase switch. The phase switch operates under the control of an external signal to either pass an incoming signal substantially unaffected (00 of phase shift) or to invert the incoming signal (1800 of phase shift).

The reason that such transmit/receive switch circuitry has limited bandwidth performance, is the limited bandwidth performance of such phase switches.

It is an object of the present invention to provide a phase switch which ameliorates this disadvantage.

According to the present invention there is provided a phase switch comprising a slotline defined by a ground plane, first microstrip means arranged to cross the slotline, means for connecting the first microstrip means to the ground plane on a first side of the slotline, second microstrip means arranged to cross the slotline, means for connecting the second microstrip means to the ground plane on a second side of the slotline opposite the first side, first switching means for switching the first microstrip means to couple a signal between the first microstrip means and the slotline, and second switching means for switching the second microstrip means to couple a signal between the second microstrip means and the slotline.

The invention provides wideband operation with low insertion loss, high power handling capability and negligible intermodulation and harmonic distortion. Particularly good performance is obtained when PIN diodes are used as the switching means.

The present invention is based upon an understanding of the behaviour of signals when transferring (or coupling) between a stripline (or microstrip) transmission line and a slot line. Stripline transmission line comprises a conductor and two ground planes, above and below the conductor respectively. The ground planes are isolated from the conductor by a dielectric layer, possibly air but usually a synthetic dielectric such as RT/Duroid (a registered Trade Mark of the ROGERS Corporation). Microstrip line is similar but comprises only one ground plane. Slotline, on the other hand, comprises a narrow gap in a ground plane or two parallel gaps (image of each other with respect to the stripline layer) on both ground planes, in the case of a stripline implementation. Signals propagating along a microstrip line or stripline (hereinafter referred to collectively as microstrip means) can be coupled into (or out of) a slotline when the microstrip means traverses the slotline. The microstrip means must be grounded on one side of the slotline. The phase of the signal coupled into a microstrip means from a slotline (or vice versa) is dependent upon which side of the slotline the microstrip means is earthed. This is a broad band phenomenon having low phase dispersion.

The present invention may thus be used to provide a phase switch in accordance with the object of the invention. The second and first microstrip means may be coupled together on the opposite sides from which they may be grounded to the slotline ground plane. Since the first and second microstrip means are grounded on opposite sides of the slotline, a signal on these coupled lines can be arranged to provide 0 of phase shift (for example by activating the first switching means) or 1800 of phase shift (for example by activating the second switching means). If the incoming signal is provided on a microstrip line or stripline then a means for coupling the signal to the slotline can comprise further microstrip means arranged to cross the slot line.

Further preferred features of the invention are defined in the accompanying claims.

While the present invention is described with reference to GSM900 and DCS1800 wavebands, it is more generally applicable to dual mode RF architectures. For example, GSM900 and DECT, GSM900 and DCS1900, and GSM and Intermediary Circular Orbit (ICO) and so on.

ICO relates to a satellite communication network which will be launched in the next few years.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a slotline and stripline arrangement helpful in describing the operation of the present invention; Figure 2 shows a schematic diagram of a phase switch in accordance with the present invention; Figures 3(a) and (b) show a practical realization of the present invention for use in the GSM and PCN wavebands; Figure 4 shows a graph of reflection coefficient against frequency for the phase switch shown in Figure 3; Figure 5 shows a graph of reflection coefficient against frequency for the phase switch (in the 180C mode) shown in Figure 3; Figure 6 shows the relative phase between the two switching states of the phase switch shown in Figure 3; and Figure 7 shows a graph of relative amplitude against frequency between the two switching states of the phase switch shown in Figure 3.

Figure 1 shows a perspective view of a first portion 10 and a second portion 12 of a ground plane which defines a slotline 14. Above the ground plane, and crossing the slot line 14 are a first microstrip line 16 and a second microstrip line 18. The microstrip lines are arranged above the ground plane with a finite separation therefrom. Microstrip line is usually supported with respect to the ground plane by a dielectric material (not shown for clarity). The first microstrip line 16 is shorted to the portion 10 of the ground plane by a conducting stud (or "via") 20.

The second microstrip line 18 is connected to the second portion 12 of the ground plane with a via 22. One aspect of the behaviour of this arrangement is as follows.

If a microwave input signal is applied at point X to the portion of the microstrip line 16 distant from the via 20, this signal will be coupled into the slotline 14. The exact mechanism by which this occurs is known, but is complex and beyond the scope of the present description. The behaviour of microstrip line 18 is the reverse of that of stripline 16. A signal Y taken from the end of the microstrip line 18 distant from the via 22 provides a signal which has been coupled-out of the slotline 14.

Because the stripline 18 is grounded on the opposite side of the slotline 14 from the grounding of the microstrip line 16, the signal Y taken from the microstrip line 18 is in anti-phase (or has a 1800 phase shift) with respect to the input signal X. The present invention exploits this phenomenon by providing switching means to derive signals either in phase or out-of-phase from the slotline 14.

Figure 2 shows a schematic diagram of a phase switch 30 in accordance with the invention. A ground plane 32 is shown which may extend further in any or all directions. Within the ground plane is provided a slot line 34 which is open (e.g. 36) at each end. The oval gap 36 in the ground plane is a means of illustrating that the slotline is open (or effectively so).

An input port 38 of the circuit is coupled to a microstrip (or stripline) 40 which crosses the slotline 34 and is earthed to the ground plane on the right-hand side of the slotline at 42. A microstrip line (or stripline) 44 crosses the slotline 34 and coupled in succession to diode 46 and earth 48. A microstrip line (or stripline) 50 crosses the slotline 34 and is coupled in succession to diode 52 and earth at 54. The diodes 46 and 52 may comprise PIN diodes. The earth 48 of microstrip line 44 is on the opposite side of the slotline from the earth 42 while the earth 54 of the microstrip line 50 is on the same side of the slotline 34 as the earth 42. The microstrip lines 44, 50 are combined together to provide an output port 56.

Some switching circuitry is required in order to switch the diodes 46, 52 on and off. This circuitry, however, has been omitted from the present diagram for reasons of clarity. It will be discussed in more detail with reference to Figure 3 later. The operation of the phase switch shown in Figure 2 is as follows.

A signal is imagined to be applied to the input port 38. This signal propagates along the line 40 which is earthed at 42. Since the line 40 crosses the slotline 34 and is earthed on one side thereof, the input signal is coupled into the slotline 34. If we imagine that diode 52 is conducting and diode 46 is off, then a signal is coupled into the line 50.

The line 50 is a microstrip line (or stripline) which crosses the slotline 34 and is earthed 54 on one side thereof. As discussed with reference to Figure 1, a signal will be coupled from the slotline 34 to the microstrip line 50. Since the line 40 and the line 50 are earthed at 42 and 54 on the same side of the slotline 34, the output signal at 56 will be in-phase with the input signal at 38 (00 of phase shift). Since the diode 46 is off, no signal is coupled into the microstrip line 44. The output signal from the port 56 is thus an in-phase signal.

In the opposite circumstances, the diode 52 is off while the diode 46 is on. A signal is still imagined to be applied to the input port 38. The signal is coupled into the slotline 34 as described previously but instead of being coupled out of the slotline by the microstrip line 50, it is coupled out by the micro stripline 44. Since diode 52 is off, no signal is coupled into the microstrip line 50 and the signal on microstrip line 44 is coupled to the output 56. The microstrip line 44 is earthed on the opposite side of the slotline 34 from the earth 42 of the microstrip line 40. As a consequence, the signal coupled into the microstrip line 44 from the slot line 34 is in anti-phase with the input signal applied to the micro strip line 40.

Consequently, simply by switching either diode 46 or diode 52 on, a 0 or 1800 phase shift can be provided. This phase shift gives good broadband performance over an octave of frequency (see experimental results discussed later).

Figure 3 shows an actual implementation of the present invention for the frequencies discussed above. Figure 3(a) shows a plan view showing the components of the arrangement, while Figure 3(b) shows a side view to indicate which components are arranged on which level. The arrangement 60 includes an RF input port 62 connected to a first plate of a capacitor 64. The capacitor typically has a capacitance of between 33 picofarads and 68 picofarads (68 pF). This capacitor is to isolate an input signal applied to port 62 from the DC signals discussed later. A second plate of the capacitor 64 is connected to a via V4 which is a conducting pin extending substantially in the plane of the paper for connecting the capacitor 64 to a microstrip line 66 which is on a different level within the device. A first portion 68 of a conductive plane and a second portion 70 of the conductive plane comprise a slotline ground plane. A slotline 72 is defined between these two portions of ground plane. The microstrip line 66 crosses the slotline 72 and is connected to the slotline ground plane 70 by means of a via V1. The microstrip line 66 thus couples an input signal into the slotline 72. A further microstrip line 74 crosses the slotline 72 and is attached at one end to an anode of a diode 76 and at another end to a resistor 78 and a first plate of a capacitor 80.

The cathode of the diode 76 is connected to the slotline ground plane 68 by means of a via V3. The other end of the resistor 78 is connected to a first DC control port 82 while the other plate of the capacitor 80 is connected to a further portion of microstrip line 82. Another end of the microstrip line 83 is connected to a microstrip line 84 on a different level by means of a via V5.

A further portion of microstrip line 86 crosses the slotline 72 and is connected at one end to the anode of a diode 88 and at another end to a resistor 90 and a first plate of a capacitor 92. The cathode of diode 88 is connected to slotline ground plane 70 by means of a via V2. Another end of the resistor 90 is coupled to a second DC control port 94 while another plate of the capacitor 92 is connected to a portion of microstrip line 96. Another end of microstrip line 96 is connected to the microstrip line 84 by means of via V6. The microstrip line 84 further comprises an output RF port 98 which is coupled to the vias V5 and V6.

As stated above, not all of these components are at the same level.

Figure 3 (b) shows the five layers of the device. The layer 100 comprises the microstrip ground plane which extends completely under the circuitry of the device. The microstrip line ground plane 100 is separated from the microstrip line circuitry and from the slotline ground plane by a dielectric layer 102. In the present example the dielectric layer comprises RT/Duroid 6010 which has a dielectric constant of 10.2 and a specified loss tangent of 0.0023 at 10GHz. A layer 104 contains the microstrip line 84 and the slotline ground plane 68, 70. Above this layer is a dielectric layer 106 similar to the layer 102. Above the dielectric layer 102 is a layer 108 which carries most of the microstrip circuitry, in other words the microstrip lines 66, 74, 83, 86 and 96, the capacitors 80, 92, and the diodes 76, 88. This circuitry is connected through vias V1, V2 and V3 to the slotline ground plane 68, 70 on layer 104. The vias extend through the dielectric layer 102 to connect portions 68, 70 of the slotline ground plane to the microstrip ground plane 100 which comprises a common ground.

In operation an RF input signal is applied to port 62 and coupled out at port 98. If a voltage of 10 Vdc is applied to DC control port 82 then a current will flow through the resistor 78, the microstrip line 74, the diode 76 and the via V3 to ground. The resistor 78 has a resistance of lkOhm and is provided to limit the current through the diode 76. When a DC control voltage is applied to the port 82 the diode 76 will conduct, effectively shorting one end of the microstrip line 74 to the slotline ground plane 68. A signal from the slotline 72 will thus be coupled into the microstrip line 74 in accordance with the principles discussed above.

This signal will pass through the capacitor 80 and the microstrip line 83, via V5 and microstrip line 84 to the RF output port 98. The capacitor C typically has a capacitance of between 33pF and 68pF and is provided to isolate the DC control signal applied at port 82 from the RF output port 98. Because the microstrip line 74 is earthed to the slotline ground plane on the opposite side of the slotline 72 from the grounding of microstrip line 66 (when diode 76 is conducting), the signal available at RF output port 98 will be in anti-phase to that at RF input port 62 (1800 phase shift).

When a DC control voltage of 10 Vdc is applied to DC control port 94 a current flows via resistor 90, microstrip line 86, diode 88 and via V2 to slotline ground plane 70. The resistor 90 is provided to limit the current through the diode 88 and has a value of 1 kOhm. Thus, when is a signal is applied to RF input port 62, a signal is coupled from the slotline 72 into the microstrip line 86. This signal flows via capacitor 92 through the microstrip line 96, via V6 and microstrip line 84 to RF output port 98. The capacitor 92 is provided to isolate the RF output port 98 from the DC control signal applied at port 94. The operation is analogous to that described above for a DC control voltage applied to port 82.

However, since the microstrip line 86 is connected to the slotline ground plane 70 on the same side of the slotline 72 as the microstrip line 66, the signal at RF output port 98 will be in-phase with a signal applied to RF input port 62.

Therefore, whether the phase switch imposes no phase change (00 phase shift) or 1800 phase shift can be determined by application of control voltages to either port 94 or port 82.

The dimensions of the device shown in Figure 3 are as follows: D1 = 8 mm, D2 = 10 mm, microstrip line width = 1.73 mm.

The arrangement of Figure 3 could equally be implemented in a stripline layout. Considering Figure 3(b) another dielectric layer is required above the layer 108 and a further slotline ground plane is provided above this dielectric layer. The further slotline ground plane is an image of that on layer 104, i.e. portions 68 and 70. In this case the microstrip lines 74, 83, 86 and 96 become striplines.

Figures 4 to 7 show test results obtained from testing of the phase switch shown in Figure 3. Figure 4 shows a graph of the reflection coefficient (dB) measured at the input port, when a 50Q termination is connected to the output port. (Since the device is reciprocal the same results will be achieved if the output and input ports are interchanged.) The horizontal axis represents frequency in MHz while the vertical axis is marked in decibels (dB). The graph shows the GSM frequency band between arrows 1 and 2 while to the right of the graph, the arrows 3 and 4 show the PCN frequency band. The measured attenuations at these four points are as follows: 1. 880 MHz -10.177 dB 2. 960 MHz -11.418 dB 3. 1710 MHz -22.285 dB 4. 1880 MHz -32.377 dB Figure 5 shows a graph of reflection coefficient (dB) for the 1800 phase state (or mode) against frequency. The reflection coefficient is shown in dB on the vertical axis while the frequency is shown in MHz on the horizontal axis. The arrows 1, 2 and 3, 4 show the GSM band width and the PCN band width respectively. The actual attenuations at these four points are as follows: 1. 880 MHz 0.6515 dB 2. 960 MHz 0.6456 dB 3. 1710 MHz 0.6091 dB 4. 1880 MHz 0.7155 dB This graph shows a consistently low level of attenuation across all of the frequency bands of interest.

Figure 6 shows the relative phase between the 0 and 1800 phase switching modes against frequency. The vertical axis represents degrees while the horizontal axis is calibrated in MHz. The curve actually shows the difference in phase between the output in the 0 phase state subtracted from the output in the 1800 (anti-phase) state. It can be seen that the relative phase is very close to 1800. The variation of the phase difference does not exceed 5 over an octave of bandwidth. If desired, this small offset of the phase difference from 1800 can be compensated with a small section of microstrip line. The four arrows 1, 2 and 3, 4 delineate the GSM and PCN frequency bands as before. The actual relative phase changes at these four points are as follows: 1. 880 MHz 173.340 2. 960 MHz 173.310 3. 1710 MHz 171.390 4. 1880 MHz 175.860 This level of phase change is well within acceptable limits for use in a transmit/receive switch as discussed previously.

Figure 7 shows a graph of the relative amplitude of the output of the phase switch in the 0 state and the 1800 state. As before, the vertical axis is marked in decibels and the horizontal axis is marked in MHz.

The four arrows 1,2 and 3,4 delineate the GSM and PCN wave bands as discussed previously. The actual variation in amplitude between the two states at these four points is: 1. 880 MHz 0.2418 dB 2. 960 MHz -0.2878 dB 3. 1710 MHz -0.0939 dB 4. 1880 MHz 0.3682 dB These variations are very small and well within acceptable limits.

The phase switch described herein can be applied to hybrid combiners in addition to transmit/receive switches and used in a double balanced mixer, a gyrator or isolator. The invention is not restricted to the embodiment described since this can be modified to utilise different components, different sizes (for example to accommodate different frequency ranges) and different materials as is known to those skilled in the art. Possible variations include different switching devices (e.g. FET's or bipolar transistors which provide better response but may be more expensive) and the use of a stripline arrangement rather than microstrip.

The polarity of the switching diodes and the polarity of the control signal applied thereto could both be reversed. The in-phase and anti-phase portions of stripline may be reversed with respect to input microstrip line (66, Figure 3(a)). The present invention also encompasses any novel feature disclosed herein, implicitly or explicitly, as will be apparent to the skilled person.

Claims (11)

1. A phase switch comprising a slotline defined by a ground plane, first microstrip means arranged to cross the slotline, means for connecting the first microstrip means to the ground plane on a first side of the slotline, second microstrip means arranged to cross the slotline, means for connecting the second microstrip means to the ground plane on a second side of the slotline opposite the first side, first switching means for switching the first microstrip means to couple a signal between the first microstrip means and the slotline, and second switching means for switching the second microstrip means to couple a signal between the second microstrip means and the slotline.

2. A phase switch as claimed in claim 1, further comprising means for coupling a signal to the slotline.

3. A phase switch as claimed in claim 2, wherein the means for coupling a signal to the slotline comprises a further microstrip means arranged to cross the slotline.

4. A phase switch as claimed in claim 1, claim 2 or claim 3, wherein the first microstrip means on the second side of the slotline and the second microstrip means on the first side of the slotline are coupled together.

5. A phase switch as claimed in claim 4, wherein the first microstrip means on the second side of the slotline and the second microstrip means on the first side of the slotline are coupled together with means for coupling a signal to them.

6. A phase switch as claimed in any one of claims 1 to 5, wherein the first and second switching means comprise diodes.

7. A phase switch as claimed in claim 6, further comprising a resistor connected to each diode and wherein each diode is controlled by DC voltage applied via the respective resistor.

8. A phase switch as claimed in claim 6 or claim 7, wherein the diodes comprise PIN diodes.

9. A phase switch as claimed in any one of the claims 1 to 8, wherein the microstrip means comprise striplines.

10. A phase switch as claimed in any one of the claims 1 to 8, wherein the microstrip means comprise microstrip lines.

11. A phase switch substantially as hereinbefore described with reference to the accompanying drawings.